It has been estimated that some current large commercial transport aircraft may be able to save from 15 to 20 gallons of fuel per year for every pound of weight that can be saved when building the aircraft. Over the projected 20 year life of an airplane, this savings amounts to 300 to 400 gallons of fuel. At current fuel costs, a significant investment to reduce the structural weight of the aircraft can be made to improve overall economic efficiency of the aircraft.
The need for improved performance in aircraft of various types can be satisfied by the use of improved engines, improved airframe design, or by the use of new or improved structural materials. Improvements in engines and aircraft design have been vigorously pursued, but only recently has the development of new and improved structural materials received commensurate attention, and their implementation in new aircraft designs is expected to yield significant gains in performance.
Materials have always played an important role in dictating aircraft structural concepts. Since the early 1930's, structural materials for large aircraft have remained remarkably consistent, with aluminum being the primary material of construction in the wing, body and empennage, and with steel being utilized for landing gears and certain other speciality applications requiring very high strength. Over the past several years, however, several important new materials concepts have been under development for incorporation into aircraft structures. These include new metallic materials, metal matrix composites and resin matrix composites. It is believed by many that improved aluminum alloys and carbon fiber resin matrix will dominate aircraft structural materials in the coming decades. While composites will be used in increased percentages as aircraft structural materials, new lightweight aluminum alloys, and especially aluminum-lithium alloys show great promise for extending the usefulness of materials of this type.
Heretofore, aluminum-lithium alloys have been used only sparsely in aircraft structures. The low use has been caused by their relatively low fracture toughness and by casting difficulties associated with lithium-bearing aluminum alloys compared to other more conventional aluminum alloys. Lithium additions to aluminum, however, provide a substantial lowering of the density which has been determined to be very important in decreasing the overall structural weight of aircraft. While substantial strides have been made in improving the aluminum-lithium processing technology, a major challenge is still to obtain a good blend of fracture toughness and high strength in these alloys.